The present invention relates to high-purity yttrium, a method of producing the high-purity yttrium, a sputtering target produced using the high-purity yttrium, a metal gate film mainly composed of the high-purity yttrium, and a semiconductor element and a device each comprising the metal gate film.
Yttrium (Y) is a rare earth element. Yttrium is an ash black metal having an atomic number of 39 and an atomic weight of 88.91 and has a hexagonal close-packed structure, a melting point of 1520° C., a boiling point of 3300° C., and a density of 4.47 g/cm3. Yttrium is readily oxidized on the surface in the air, is soluble in acid, but insoluble in alkali, and reacts with hot water. Its ductility and extensibility are low (see Dictionary of Physics and Chemistry).
Rare earth elements having an oxidation number of 3 are generally stable, and yttrium is trivalent. Recently, yttrium has been researched and developed as an electronic material such as a metal gate material or a high-dielectric constant (high-k) material and is a metal attracting a lot of attention.
An yttrium metal has a problem of being easily oxidized during purification and is therefore a material of which high-purification is difficult and there was not a high-purity product. An yttrium metal that has been left to stand in the air is oxidized within a short period of time into Y2O3 and changes the color into black.
Recently, there is a demand for reducing the thicknesses of gate insulating films of next-generation MOSFETs, but in SiO2, which has been used for gate insulating films, the leak current due to a tunnel effect increases with a reduction in thickness, resulting in a difficulty in normal operation.
Accordingly, as alternatives thereto, HfO2, ZrO2, Al2O3, and La2O3 have been proposed as materials having high dielectric constants, high thermal stability, and high energy barriers to holes and electrons in silicon. In these materials, in particular, La2O3 is highly rated and has been investigated for its electrical characteristics, and studies on La2O3 as a material for gate insulating films in next-generation MOSFETs have been reported (see Non-Patent Document 1). However, the study in this Non-Patent Document relates to La2O3 films and does not particularly mention the characteristics and behaviors of an yttrium (Y) element itself.
Thus, lanthanum is a material that is gathering attention in a tendency of recent technologies, but yttrium, which is a metal having similar physical properties as a rare earth metal, has almost not been studied for its use as an electronic part material. It is easily supposed that if yttrium is used in such an electronic part (e.g., a gate insulating film of next-generation MOSFET), the presence of other impurities is undesirable for taking advantage of the characteristics of yttrium itself as a metal having physical properties as a rare earth metal, and an increase in purity is necessary.
Thus, yttrium (yttrium oxide) is still in the research phase. In investigation of the characteristics of yttrium (yttrium oxide), if an yttrium metal itself is present as a sputtering target material, the sputtering target has such considerable advantages that: a thin film of yttrium can be formed on a substrate; the behaviors of the interface with a silicon substrate can be readily investigated; the characteristics of, for example, a gate insulating film having a high dielectric constant can be readily investigated by forming an yttrium compound; and also the degree of freedom as a product increases.
In formation of a film by sputtering with a target of yttrium, occurrence of a protrusion (nodule) on a target surface is a problem. The protrusion induces abnormal discharge to cause generation of particles by, for example, rupture of the protrusion (nodule).
The generation of particles causes increases in failure rates of metal gate films and semiconductor elements and devices. Accordingly, in order to utilize the characteristics of yttrium, reductions in contents of, in particular, Al, Fe, and Cu are required. In addition, carbon (graphite) contained in yttrium is present as a solid and is difficult to be detected because of it conductivity. The amount of carbon is therefore required to be reduced.
Furthermore, yttrium is a material of which high purification is difficult, but in order to utilize the characteristics of yttrium, in addition to Al, Fe, Cu, and carbon (graphite), the amounts of materials that affect the characteristics of semiconductors, such as alkaline metals, alkali earth metals, transition metal elements, high-melting-point metal elements, and radioactive elements, are required to be reduced. Accordingly, yttrium is desired to have a purity of 5N or more.
Furthermore, there is a problem that removal of lanthanoids other than yttrium is significantly difficult. Fortunately, since lanthanoids other than yttrium have similar properties, slight contamination thereof does not cause any problem. In addition, slight contamination of gas components does not cause a big problem. Removal of gas components is usually difficult, and purities are generally shown as those excluding gas components.
Conventionally, the problems related to characteristics of yttrium, how to produce high-purity yttrium, and behaviors of impurities contained in an yttrium target have not sufficiently investigated. Accordingly, it is desired to immediately solve the problems mentioned above.
In publicly known documents, Patent Document 1 describes a molten salt electrolysis apparatus that can be installed to a vacuum distillation apparatus as an apparatus for producing high-purity yttrium. In this case, however, it is unclear how highly purified yttrium can be produced.
Patent Document 2 discloses, as a method of producing high-purity yttrium, a method in which machinery arrangement of a molten salt electrolysis apparatus and a vacuum distillation apparatus has been devised, and proposes subjecting the high-purity yttrium to electron beam melting thereafter. In addition, an example of reducing each concentration of Fe, Cr, Ni, U, and Th as impurities of interest to less than 1 ppm is shown. However, it is not clearly described how much the impurities can be reduced in each step, what becomes of other impurities, and how much high purity is eventually achieved.
Patent Document 3 describes a molten salt electrolysis apparatus in which the structure of a crucible has been devised for a method of producing high-purity yttrium. In addition, an example of yttrium in which each concentration of Fe, Cr, Ni, Cu, U, and Th as impurities of interest is reduced to less than 1 ppm is shown. However, it is unclear how highly purified yttrium can be produced and how impurities other than the above are removed from yttrium.
Patent Document 4 describes a molten salt electrolysis apparatus in which the structures of an anode and a crucible have been devised for a method of producing high-purity yttrium. In addition, an example of yttrium in which each concentration of Fe, Cr, Ni, Cu, U, and Th as impurities of interest is reduced to less than 1 ppm is shown. However, it is unclear how highly purified yttrium can be produced and how impurities other than the above are removed from yttrium.
Patent Document 5 describes a vacuum distillation apparatus for yttrium chloride anhydrous in which the arrangement structure of a distillation container and a condenser is devised for a method of producing high-purity yttrium. In addition, an example of yttrium in which each concentration of Fe, Cr, Ni, Cu, Mg, and Mn as impurities of interest is reduced to less than 1 ppm is shown. However, it is unclear how highly purified yttrium can be eventually produced and how impurities other than the above are removed from yttrium.
Patent Document 6 describes use of an amorphous film of yttrium for forming a YAG thin film that is used as a solid laser oscillation material. High-purity yttrium is probably used, but the purity of this yttrium and the technology for producing this high-purity yttrium are not disclosed.
Patent Document 7 describes a solvent extraction process as a method of separating high-purity yttrium. It is described that the resulting purity of a Y compound relative to all rare earth compounds reaches 99.0% to 99.996% (wt %). However, it is not clearly described what becomes of other impurities such as transition metals and how much high purity is achieved on the whole.    Patent Document 1: Japanese Patent Laid-Open No. H04-176886    Patent Document 2: Japanese Patent Laid-Open No. H04-176887    Patent Document 3: Japanese Patent Laid-Open No. H04-176888    Patent Document 4: Japanese Patent Laid-Open No. H04-176889    Patent Document 5: Japanese Patent Laid-Open No. H05-17134    Patent Document 6: Japanese Patent Laid-Open No. H07-126834    Patent Document 7: Japanese Patent Laid-Open No. 2004-36003    Non-Patent Document 1: Eisuke Tokumitsu and two others, “Study of oxide materials for high-k gate insulating film”, Denki Gakkai Denshi Zairyo Kenkyukai Shiryo, Vol. 6-13, pp. 37-41, published on 21 Sep. 2001